On Wed, Jan 28, 2015 at 10:03:35AM -0800, Paul E. McKenney wrote:
> On Tue, Jan 27, 2015 at 11:55:08PM -0200, Marcelo Tosatti wrote:
> > On Tue, Jan 27, 2015 at 12:37:52PM -0800, Paul E. McKenney wrote:
> > > On Mon, Jan 26, 2015 at 02:14:03PM -0500, Luiz Capitulino wrote:
> > > > Paul,
> > > >
> > > > We're running some measurements with cyclictest running inside a
> > > > KVM guest where we could observe spinlock contention among rcuc
> > > > threads.
> > > >
> > > > Basically, we have a 16-CPU NUMA machine very well setup for RT.
> > > > This machine and the guest run the RT kernel. As our test-case
> > > > requires an application in the guest taking 100% of the CPU, the
> > > > RT priority configuration that gives the best latency is this one:
> > > >
> > > > 263 FF 3 [rcuc/15]
> > > > 13 FF 3 [rcub/1]
> > > > 12 FF 3 [rcub/0]
> > > > 265 FF 2 [ksoftirqd/15]
> > > > 3181 FF 1 qemu-kvm
> > > >
> > > > In this configuration, the rcuc can preempt the guest's vcpu
> > > > thread. This shouldn't be a problem, except for the fact that
> > > > we're seeing that in some cases the rcuc/15 thread spends 10us
> > > > or more spinning in this spinlock (note that IRQs are disabled
> > > > during this period):
> > > >
> > > > __rcu_process_callbacks()
> > > > {
> > > > ...
> > > > local_irq_save(flags);
> > > > if (cpu_needs_another_gp(rsp, rdp)) {
> > > > raw_spin_lock(&rcu_get_root(rsp)->lock); /* irqs disabled. */
> > > > rcu_start_gp(rsp);
> > > > raw_spin_unlock_irqrestore(&rcu_get_root(rsp)->lock, flags);
> > > > ...
> > >
> > > Life can be hard when irq-disabled spinlocks can be preempted! But how
> > > often does this happen? Also, does this happen on smaller systems, for
> > > example, with four or eight CPUs? And I confess to be a bit surprised
> > > that you expect real-time response from a guest that is subject to
> > > preemption -- as I understand it, the usual approach is to give RT guests
> > > their own CPUs.
> > >
> > > Or am I missing something?
> >
> > We are trying to avoid relying on the guest VCPU to voluntarily yield
> > the CPU therefore allowing the critical services (such as rcu callback
> > processing and sched tick processing) to execute.
>
> These critical services executing in the context of the host?
> (If not, I am confused. Actually, I am confused either way...)
The host. Imagine a Windows 95 guest running a realtime app.
That should help.
> > > > We've tried playing with the rcu_nocbs= option. However, it
> > > > did not help because, for reasons we don't understand, the rcuc
> > > > threads have to handle grace period start even when callback
> > > > offloading is used. Handling this case requires this code path
> > > > to be executed.
> > >
> > > Yep. The rcu_nocbs= option offloads invocation of RCU callbacks, but not
> > > the per-CPU work required to inform RCU of quiescent states.
> >
> > Can't you execute that on vCPU entry/exit? Those are quiescent states
> > after all.
>
> I am guessing that we are talking about quiescent states in the guest.
Host.
> If so, can't vCPU entry/exit operations happen in guest interrupt
> handlers? If so, these operations are not necessarily quiescent states.
vCPU entry/exit are quiescent states in the host.
> > > > We've cooked the following extremely dirty patch, just to see
> > > > what would happen:
> > > >
> > > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > > > index eaed1ef..c0771cc 100644
> > > > --- a/kernel/rcutree.c
> > > > +++ b/kernel/rcutree.c
> > > > @@ -2298,9 +2298,19 @@ __rcu_process_callbacks(struct rcu_state *rsp)
> > > > /* Does this CPU require a not-yet-started grace period? */
> > > > local_irq_save(flags);
> > > > if (cpu_needs_another_gp(rsp, rdp)) {
> > > > - raw_spin_lock(&rcu_get_root(rsp)->lock); /* irqs disabled. */
> > > > - rcu_start_gp(rsp);
> > > > - raw_spin_unlock_irqrestore(&rcu_get_root(rsp)->lock, flags);
> > > > + for (;;) {
> > > > + if (!raw_spin_trylock(&rcu_get_root(rsp)->lock)) {
> > > > + local_irq_restore(flags);
> > > > + local_bh_enable();
> > > > + schedule_timeout_interruptible(2);
> > >
> > > Yes, the above will get you a splat in mainline kernels, which do not
> > > necessarily push softirq processing to the ksoftirqd kthreads. ;-)
> > >
> > > > + local_bh_disable();
> > > > + local_irq_save(flags);
> > > > + continue;
> > > > + }
> > > > + rcu_start_gp(rsp);
> > > > + raw_spin_unlock_irqrestore(&rcu_get_root(rsp)->lock, flags);
> > > > + break;
> > > > + }
> > > > } else {
> > > > local_irq_restore(flags);
> > > > }
> > > >
> > > > With this patch rcuc is gone from our traces and the scheduling
> > > > latency is reduced by 3us in our CPU-bound test-case.
> > > >
> > > > Could you please advice on how to solve this contention problem?
> > >
> > > The usual advice would be to configure the system such that the guest's
> > > VCPUs do not get preempted.
> >
> > The guest vcpus can consume 100% of CPU time (imagine a guest vcpu busy
> > spinning). In that case, rcuc would never execute, because it has a
> > lower priority than guest VCPUs.
>
> OK, this leads me to believe that you are talking about the rcuc kthreads
> in the host, not the guest. In which case the usual approach is to
> reserve a CPU or two on the host which never runs guest VCPUs, and to
> force the rcuc kthreads there. Note that CONFIG_NO_HZ_FULL will do this
> automatically for you, reserving the boot CPU. And CONFIG_NO_HZ_FULL
> might well be very useful in this scenario. And reserving a CPU or two
> for housekeeping purposes is quite common for heavy CPU-bound workloads.
>
> Of course, you need to make sure that the reserved CPU or two is sufficient
> for all the rcuc kthreads, but if your guests are mostly CPU bound, this
> should not be a problem.
>
> > I do not think we want that.
>
> Assuming "that" is "rcuc would never execute" -- agreed, that would be
> very bad. You would eventually OOM the system.
>
> > > Or is the contention on the root rcu_node structure's ->lock field
> > > high for some other reason?
> >
> > Luiz?
> >
> > > > Can we test whether the local CPU is nocb, and in that case,
> > > > skip rcu_start_gp entirely for example?
> > >
> > > If you do that, you can see system hangs due to needed grace periods never
> > > getting started.
> >
> > So it is not enough for CB CPUs to execute rcu_start_gp. Why is it
> > necessary for nocb CPUs to execute rcu_start_gp?
>
> Sigh. Are we in the host or the guest OS at this point?
Host.
> In any case, if you want the best real-time response for a CPU-bound
> workload on a given CPU, careful use of NO_HZ_FULL would prevent
> that CPU from ever invoking __rcu_process_callbacks() in the first
> place, which would have the beneficial side effect of preventing
> __rcu_process_callbacks() from ever invoking rcu_start_gp().
>
> Of course, NO_HZ_FULL does have the drawback of increasing the cost
> of user-kernel transitions.
We need periodic processing of __run_timers to keep timer wheel
processing from falling behind too much.
See http://www.gossamer-threads.com/lists/linux/kernel/2094151.
> > > Are you using the default value of 16 for CONFIG_RCU_FANOUT_LEAF?
> > > If you are using a smaller value, it would be possible to rework the
> > > code to reduce contention on ->lock, though if a VCPU does get preempted
> > > while holding the root rcu_node structure's ->lock, life will be hard.
> >
> > Its a raw spinlock, isnt it?
>
> As I understand it, in a guest OS, that means nothing. The host can
> preempt a guest even if that guest believes that it has interrupts
> disabled, correct?
Yes.
> If we are talking about the host, then I have to ask what is causing
> the high levels of contention on the root rcu_node structure's ->lock
> field. (Which is the only rcu_node structure if you are using default
> .config.)
>
> Thanx, Paul
OK, great.
Thanks a lot.
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